Mirror modifier problem for only one cube
So from what I get from you question is that you are trying to model a low poly car with the mirror modifier activated. but you can't seem to apply it or add extra meshes?

To be able to apply any modifier you need to be in object mode.


So you would first create a cube and from front view in edit mode with "Control + R" create an edge loop in the middle

after that you want to delete the first set of vertices on any side.
And then apply the mirror modifier with "Clipping" checked 



Then you can edit the mesh however you like and you have 2 ways of adding new meshes 


While in edit mode press "shift + a" to create a new mesh
Go to object mode, press "shift + a" to create a new mesh and repeat
steps 1 / 3 with the new model.